Wireless communication devices, such as radiotelephones, which are adapted to communicate with a base station, such as a cordless telephone base station, typically have a predetermined set of channels on which they can transmit and receive radio frequency (RF) signals. A wireless communication device must determine which of the channels are not occupied by another device and are clear to use. Conventional communication devices have generated a prioritized list of available channels. This prioritized list could be based upon the most recently observed radio frequency activity on the given channels. For example, when scanning a list of channels, any channel having interference would be placed at the bottom of the list. Such conventional schemes merely create an ordered list of channels as they are scanned, with channels having interference being placed at the bottom of the list. Other schemes of interference avoidance have been developed wherein each channel of a range of channels is periodically sampled. The system records the number of times interference is detected for each channel over a period of time. A probability of interference for each channel can then be calculated.
However, such methods have significant limitations. For example, a channel at or near the top of the list may not have been clear for a minimum threshold period of time, and therefore, may not be an acceptable channel for communications. Also, when assigning the channel, such conventional systems will typically assign the first available channel on the list, resulting in considerable problems of collision avoidance. For example, two communication devices which are powered up at the same time may select the same channel because they are both employing the same channel selection scheme. This problem would also exist when multiple communication devices are located near each other. Such communication devices running for a period of time would be expected to have similar interference information for channels used by the surrounding system. Also, if the channel at the top of the list is adjacent (i.e., the next sequential channel in frequency) to the channel which is receiving interference, the channel at the top of the list may also be receiving interference. In a system which includes in-call interference detection and channel change, an adjacent channel interferer with a sufficiently strong signal will appear as interference, causing the system to decide to change channels. If the system had no memory of interference on this adjacent channel, it is possible that this adjacent channel would be chosen in the channel change because it is the next sequential channel number. This would cause the communication device to change channels to the channel actually causing the interference.
Accordingly, there is a need for a method and apparatus assigning a channel to a communication device to avoid interference.